1. Field of the Invention
The present invention relates to a discharge cell used in a discharge type ozonizer.
2. Description of the Related Art
Discharge cells used in a discharge type ozonizer called an ozonizer are roughly divided into plate types and tube types. Either type of the discharge cell has a pair of electrodes arranged with a gap between them, and has a configuration in which a dielectric is arranged between the electrodes contacting the surface of at least one of the pair of the electrodes to form a discharge gap between the electrodes. Then ozone gas is produced by circulating a raw material gas such as oxygen in a state that a fixed high-frequency high-voltage is applied to the discharge gap and silent discharge is generated.
Recently, in order not to expose the electrodes made of metal to the discharge gap, a configuration has been increasingly emerging to form a discharge gap between a pair of dielectrics by arranging a pair of dielectrics inside each of the paired electrodes. Furthermore, a multi-layered structure has been used a lot in which this configuration is considered as one unit, and a plurality of the units are layered in the direction of the thickness.
The dielectric in the discharge cell is roughly divided by shape into a substrate type having rigidity and a coated type in which the surface of the gap side of the rigid electrodes is coated. In the case of the coated type, nonuniformity of the thickness distribution cannot be avoided, there is a problem that it leads to nonuniformity of the gap amount of the discharge gap, and the substrate type such as a ceramic plate that is hard and strong chemically has recently become mainstream.
Incidentally, an ozonizer is used in various chemical treatment equipments on the one hand and began to be used broadly in semiconductor manufacturing equipment. In the case of an ozonizer for semiconductor manufacturing used in formation of an oxide film, ashing of a resist, cleaning of a silicon wafer, etc., because high cleanliness is demanded, it is necessary to generate pure ozone gas with extremely small contamination (metal impurities and particles), and because of this, oxygen of high purity is used as the raw material gas.
Further, an alumina substrate of high purity that has a high mechanical strength and that is excellent in ozone resistance, sputtering resistance, etc., is recommended as a substrate type dielectric in the respect of securing cleanliness. Furthermore, from the necessity of producing ozone gas of high concentration, reducing and uniformizing the gap amount in the discharge gap are attempted together with high purification of the oxygen gas, and a gap amount that has been reduced to 0.2 mm or less can be seen currently.
In the case of using oxygen gas of high purity as the raw material gas, it has already widely been known that there is a problem that the ozone concentration of ozone gas rapidly decreases right after the operation is started and the prescribed performance does not appear. In order to solve the problem, it is considered to be effective to add a small amount of catalyst gas to the oxygen gas of high purity, and nitrogen gas of high purity that is easily obtained in a semiconductor manufacturing process has been used a lot as the catalyst gas.
The case that the dielectric is the above-described high purity alumina substrate is also not an exception, and in the case that the raw material gas is oxygen gas of high purity, the performance as an ozonizer hardly appears. Furthermore, in the case of the high purity alumina substrate, it has been clear that the ozone concentration does not increase sufficiently even when nitrogen gas is mixed to the oxygen gas. In more detail, in the case of arranging the high purity alumina substrate on both faces of the discharge gap, especially the effect of adding the catalyst gas cannot be obtained sufficiently. This is considered to be because impurities are removed extremely from the surface of the dielectric contacting the discharge gap.
From these circumstances, an attempt to increase the ozone concentration without using a catalyst gas has proceeded in various fields, and one of them is the use of titanium oxide to the dielectric described in Japanese Patent Application Laid-Open (JP-A) No. 11-21110 and JP-A No. 2005-350336, etc. Further, a coating of a tungsten-based substance to the surface of the dielectric has been considered as shown in U.S. Pat. No. 5,932,180 and JP-A No. 2005-320223.
By coating the surface of the dielectric with the titanium oxide described in Japanese Patent Application Laid-Open (JP-A) No. 11-21110 and JP-A No. 2005-350336 or incorporating it in the entire dielectric, the concentration of the ozone gas can be increased. However, in the case that the purity of the oxygen gas, which is the raw material gas, is 99.9% or more, the ozone concentration does not increase without adding nitrogen gas. However, by adding nitrogen gas at a small amount (for example, about 0.5 vol %), the ozone concentration increases rapidly, and it becomes possible to draw out the ability of an ozonizer to the fullest. That is, titanium oxide has the effect of reducing the amount of nitrogen gas used. However, the effect of increasing the ozone concentration without adding nitrogen gas is small.
On the other hand, concerning the tungsten-based substance, the surface of the dielectric is coated with metal tungsten in U.S. Pat. No. 5,932,180, and the surface of the dielectric contacting to the discharge gap and the surface of the electrodes are coated with tungsten oxide having a fixed electric resistivity in JP-A No. 2005-320223.
In U.S. Pat. No. 5,932,180, the surface of the dielectric is coated with metal tungsten. However, this metal tungsten changes into tungsten oxide (WO3) by a strong oxidizing-power of ozone produced in the discharge gap, and the surface of the dielectric is considered to be actually coated with this WO3. Incidentally, WO3 is an insulator.
Against this, in JP-A No. 2005-320223, the surface of the dielectric contacting to the discharge gap and the surface of the electrodes are coated with a conductive tungsten oxide having a fixed resistivity. Among the tungsten oxides, WO3 is an electrical insulator. However, the conductivity of WO2 is good and it is a fact that the resistivity of the tungsten oxide can be changed by changing the amount of oxygen. However, because this conductive tungsten oxide also contacts to the discharge gap and directly contacts to ozone, WOX (X<3) is considered to change to WO3, which is an insulator, as it is being used after all.
That is, the face contacting to the discharge gap is considered to be coated with WO3 in the discharge cell to be actually used, also in the technology described in U.S. Pat. No. 5,932,180 and the technology described in JP-A No. 2005-320223 even though the expression is different.
However, in order for the present inventors to confirm the effectiveness of WO3, when the surface of the dielectric was coated with WO3, or WO3 was compounded in the dielectric, it was confirmed that the effect of increasing the ozone concentration is substantially larger than TiO2 in the case without adding oxygen. However, it does not result in drawing out the ability of the ozonizer to the fullest. If anything, even though nitrogen gas is added in a small amount, its effect is not much improved and it does not result in drawing out the ability of the ozonizer to the fullest. That is, the ozone concentration in the case of adding a small amount of nitrogen gas becomes higher with TiO2 than WO3.